Project Summary Infectious diseases contribute three of the top ten causes of death worldwide. Moreover, billions of people remain infected with persisting pathogens such as hepatitis B virus, malaria and HIV, and ~half a million people die annually from influenza (Flu) and other respiratory infections. Much of our knowledge of how the human immune system responds to pathogens or vaccination remains observational rather than direct perturbation of specific molecules. Here, we will exploit the revolution in immunotherapy of cancer to directly interrogate how key immune regulatory pathways impact T cell, B cell and antibody (Ab) responses to human vaccination toward developing a foundation of empirical evidence that informs how to use checkpoint blockade and targeting of other specific immune pathways to treat or prevent infectious disease. T cell and B cell activation and differentiation are regulated in part by the PD-1:PD-L1/2 inhibitory receptor pathway, which also influences T cell exhaustion. These functions are targeted by a new class of biologic drugs that block PD-1 signals to treat cancer. Yet, the role of the PD-1 pathway in human immune responses to vaccination remains unclear. In particular, cells in germinal centers (GC), including follicular helper T cells (TFH), follicular regulatory T cells (TFR), and GC and memory B cells (mBc), have high expression of PD-1 and/or its ligands, but the contribution of the PD-1 pathway to GC responses and humoral, TFH and B cell memory is poorly understood. Further, despite widespread use of anti-PD-1 (?PD-1) therapy in recent years, there is essentially no information on how this treatment impacts TFH, TFR, B cell or Ab responses or whether patients on checkpoint blockade should or should not be vaccinated. Thus, the overarching hypothesis of this Project is that blocking PD-1 in humans will impact the magnitude and/or quality of the TFH, TFR, B cell and humoral response to Flu vaccination through an underlying effect on GC biology. We will: AIM 1: DETERMINE HOW ?PD-1 IMPACTS THE FUNCTIONAL DIVERSITY, MAGNITUDE, TCR REPERTOIRE, DURABILITY AND TRANSCRIPTIONAL PROGRAM OF VACCINE-INDUCED CD4 T CELL RESPONSES. AIM 2: DETERMINE HOW ?PD-1 IMPACTS THE MAGNITUDE, BCR REPERTOIRE, DURABILITY AND TRANSCRIPTIONAL PROGRAM OF VACCINE-INDUCED B CELL RESPONSES AND QUANTITY/QUALITY OF VACCINE-INDUCED AND OTHER ANTIVIRAL ANTIBODY RESPONSES. This work will inform how to better vaccinate vulnerable populations (e.g. cancer patients; the elderly), and will identify mechanisms by which the PD-1 pathway controls human immunity, providing a platform for future studies of immune perturbation in humans. By its nature Project 2 is highly interactive with Project 1 and all Cores. Core B will generate samples and this Project will use Core C and D services. Projects 1 and 2 will synergize and interact extensively to understand PD-1-based immune perturbation in humans.